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      <div class="titlepage">
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          <div>
            <h2 class="title"><a id="txnconcurrency"></a>Chapter 4. Concurrency</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="txnconcurrency.html#concurrenthandles">Which DB Handles are Free-Threaded</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="blocking_deadlocks.html">Locks, Blocks, and Deadlocks</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="blocking_deadlocks.html#locks">Locks</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="blocking_deadlocks.html#blocks">Blocks</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="blocking_deadlocks.html#deadlocks">Deadlocks</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="lockingsubsystem.html">The Locking Subsystem</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="lockingsubsystem.html#configuringlock">Configuring the Locking Subsystem</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="lockingsubsystem.html#configdeadlkdetect">Configuring Deadlock Detection</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="lockingsubsystem.html#deadlockresolve">Resolving Deadlocks</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="lockingsubsystem.html#setpriority">Setting Transaction Priorities</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="isolation.html">Isolation</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="isolation.html#degreesofisolation">Supported Degrees of Isolation</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="isolation.html#dirtyreads">Reading Uncommitted Data</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="isolation.html#readcommitted">Committed Reads</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="isolation.html#snapshot_isolation">Using Snapshot Isolation</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="txn_ccursor.html">Transactional Cursors and Concurrent Applications</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="txn_ccursor.html#cursordirtyreads">Using Cursors with Uncommitted Data</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="exclusivelock.html">Exclusive Database Handles</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="readmodifywrite.html">Read/Modify/Write</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="txnnowait.html">No Wait on Blocks</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="reversesplit.html">Reverse BTree Splits</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>

        DB offers a great deal of support for multi-threaded
            <span>
                and multi-process
            </span>
        applications even when transactions are not in use. Many of DB's
        handles are 
        thread-safe<span>, or 
        can be made thread-safe by providing the appropriate flag at handle creation time,</span> 
        and DB provides a
        flexible locking subsystem for managing databases in a concurrent
        application. Further, DB provides a robust mechanism for
        detecting and responding to 
        <span>deadlocks</span>
        . 
        All of these concepts are
        explored in this chapter.
     </p>
      <p>
        Before continuing, it is useful to define a few terms that will appear
        throughout this chapter:
     </p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>
                <span class="emphasis"><em>Thread of control</em></span>
            </p>
            <p>
                Refers to a thread that is performing work in your application.
                Typically, in this book that thread will be performing DB
                operations.
            </p>
            <p>
                Note that this term can also be taken to mean a separate process
                that is performing work — DB supports multi-process
                operations on your databases.
            </p>
          </li>
          <li>
            <p>
                <span class="emphasis"><em>Locking</em></span>
            </p>
            <p>
                When a thread of control obtains 
                access to a shared resource, it is said to be
                <span class="emphasis"><em>locking</em></span> that resource. Note that
                DB supports both exclusive and non-exclusive locks. See
                <a class="xref" href="blocking_deadlocks.html#locks" title="Locks">Locks</a> for more information.
            </p>
          </li>
          <li>
            <p>
                <span class="emphasis"><em>Free-threaded</em></span>
            </p>
            <p>
                Data structures and objects are free-threaded if they can be
                shared across threads of control without any explicit locking on
                the part of the application.  Some books, libraries, and
                programming languages may use the term
                <span class="emphasis"><em>thread-safe</em></span> for data structures or objects
                that have this characteristic. The two terms mean the
                same thing.
            </p>
            <p>
                For a description of free-threaded DB objects, see
                <a class="xref" href="txnconcurrency.html#concurrenthandles" title="Which DB Handles are Free-Threaded">Which DB Handles are Free-Threaded</a>.
            </p>
          </li>
          <li>
            <p>
                <span class="emphasis"><em>Blocked</em></span>
            </p>
            <p>
                When a thread cannot obtain a lock because some other
                thread already holds a lock on that object, the lock
                attempt is said to be <span class="emphasis"><em>blocked</em></span>. See
                <a class="xref" href="blocking_deadlocks.html#blocks" title="Blocks">Blocks</a> for more information.
            </p>
          </li>
          <li>
            <p>
                <span class="emphasis"><em>Deadlock</em></span>
            </p>
            <p>
                Occurs when two or more threads of control attempt to access conflicting resource in such a way as none
                of the threads can any longer make further progress.
            </p>
            <p>
                For example, if Thread A is blocked waiting for a resource held by Thread
                B, while at the same time Thread B is blocked waiting for a
                resource held by Thread A, then neither thread can make any
                forward progress. In this situation, Thread A and Thread B
                are said to be <span class="emphasis"><em>deadlocked.</em></span>
            </p>
            <p>
                For more information, see <a class="xref" href="blocking_deadlocks.html#deadlocks" title="Deadlocks">Deadlocks</a>.
            </p>
          </li>
        </ul>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="concurrenthandles"></a>Which DB Handles are Free-Threaded</h2>
            </div>
          </div>
        </div>
        <p>
            The following describes to what extent and under what conditions
            individual handles are free-threaded.
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    
                    
                   <code class="classname">Environment</code> 
                   <span>
                           and the DPL
                           <code class="classname">EntityStore</code>
                   </span>
                </p>
              <p>
                    Free-threaded so long as
                        
                        <span>
                            <code class="methodname">EnvironmentConfig.setThreaded()</code>
                            is set to <code class="literal">true</code>.
                        </span>
                </p>
            </li>
            <li>
              <p>
                    
                    
                   <code class="classname">Database</code> 
                   <span>
                           and the DPL
                           <code class="classname">PrimaryIndex</code>
                   </span>
                </p>
              <p>
                    Free-threaded so long as
                        
                        <span>
                                the database 
                                <span>
                                        or DPL <code class="classname">PrimaryIndex</code>
                                </span>
                                is opened in a free-threaded environment.
                        </span>
                </p>
            </li>
            <li>
              <p>
                   <code class="classname">SecondaryDatabase</code> 
                   and DPL <code class="classname">SecondaryIndex</code>
                </p>
              <p>
                    Same conditions apply as for <code class="classname">Database</code>
                    and <code class="classname">PrimaryIndex</code> handles.
                </p>
            </li>
            <li>
              <p>
                    
                    
                   <code class="classname">Cursor</code> 
                   <span>
                           and the DPL
                           <code class="classname">EntityCursor</code>
                   </span>
                </p>
              <p>
                    Cursors are not free-threaded. However, they
                    can be used by multiple threads of control so 
                    long as the application serializes access to the handle.
                </p>
            </li>
            <li>
              <p>
                   <code class="classname">SecondaryCursor</code> 
                </p>
              <p>
                    Same conditions apply as for <code class="classname">Cursor</code>
                    handles.
                </p>
            </li>
            <li>
              <p>
                    
                    
                   <code class="classname">Transaction</code> 
                    
                </p>
              <p>
                    Access must be serialized by the application across threads of control.
                </p>
            </li>
          </ul>
        </div>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
                All other classes found in the DPL
                (<code class="literal">com.sleepycat.persist.*</code>) and not
                mentioned above are free-threaded.
            </p>
          <p>
                All classes found in the bind APIs (<code class="literal">com.sleepycat.bind.*</code>) are free-threaded.
            </p>
        </div>
      </div>
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